Spin-Fluctuation Transitions in MnSi According to Electron Paramagnetic Resonance and Neutron Scattering

The experimental data on low-temperature (T < 40 K) neutron scattering and electron paramagnetic resonance for a helical magnet manganese monosilicide (MnSi) are analyzed. It is established that the smooth evolution of the parameters of spin fluctuations considered both in the conventional theory of magnetic phase transitions and in its generalization to the case of helical magnets is disturbed by the presence of spin-fluctuation transitions (SFTs) in which the amplitude of spin fluctuations and their correlation radius change sharply. In a zero magnetic field, the transition at the temperature T_c = 29 K, which is usually interpreted as the transition to a helical magnetically ordered phase, is preceded by two spin-fluctuation transitions with T₁ = 32 K and T₂ = 30.5 K. In a magnetic field of B ~ 2 T at a temperature of 29 K coinciding with T_c, another spin-fluctuation transition with the parameters characteristic for the SFT inside the magnetically ordered phase is discovered. It is shown that, as the temperature decreases, MnSi at T = T₁ undergoes the SFT with the appearance of helical fluctuations, while the appearance of a helical phase (B = 0) or a spin-polarized phase (B = 2 T) occurs at T = T₂ and is accompanied by a spin-fluctuation transition.